This application is a national phase application of International Application No. PCT/KR98/00365, which was filed on Nov. 16, 1998 and which published in English on May 27, 1999, which in turn claims priority from Korean Application No. KR 1997/60661, which was filed on Nov. 17, 1997.
(a) Field of the Invention
The present invention relates to a microporous membrane and a method for providing the same, and more particularly to a method for providing a microporous membrane having hydrophilic/hydrophobic properties and pores with uniform size and shape by irradiating energized ion particles to a polymer film under vacuum.
(b) Description of the Related Art
Currently, there are various types of microporous membranes being used as a separator in lithium battery. Conventional methods for producing these microporous membranes are classified into a wet method and dry method. These methods utilize fillers or wax with a solvent as in wet method, or without the solvent as in dry method, to produce a precursor film. Then a resulting microporous membrane is obtained by forming micro-pores in the precursor film.
There are numerous methods of forming micro-pores, such as in cold and hot stretching methods the precursor film is subjected to a stretching process, and in an extraction method low molecular weight particles are extracted from the precursor film which has been subjected to a biaxial stretching (alternatively, biaxial stretching process can be implemented after the extraction method) to form micro-pores on the precursor film. Further, the precursor film can be subjected to a corona discharge method followed by a stretching, or it can be etched after being irradiated with high-energy ion-beams as in a track-etching method to obtain microporous membrane. The method utilizing cold or hot stretching process is referred to as a dry process. U.S. Pat. Nos. 3,679,538;1 3,801,692 3,843,761; 4,238,459; and 5,013,439 disclose the dry process, while U.S. Pat. Nos. 3,471,597 and 3,880,966 disclose corona discharge process for obtaining a precursor film with pores.
The dry process has an advantage in that it does not utilize environmental hazardous solvents, and hence the method is referred to as a clean process and is widely used in the industry. However, microporous membranes produced by the dry process have pores with undesirable small sizes, and presents the difficulties of adjusting and increasing shape and size of the pores. Further, there is a drawback in that during stretching, maintaining shape of the pores becomes difficult as stretch ratio increases.
The conventional methods for producing microporous mebranes to be used as a separator in lithium battery utilize polyolefin resin because of its cost and chemical and physical property. However, due to the hydrophobicity of the polyolefin resin, there is a low wettability of electrolytes for the separator. Currently, there are numerous researches being carried out to incorporate hydrophilic property to polyolefin resin membranes. The method described by Hoechst Celenese processes the surface of the polyolefin resin membrane with surfactants, and other methods described by U.S. Pat. Nos. 3,231,530; 3,853,601; 3,951,815; 4,039,440; and 4,340,482 integrates monomers having high hydrophillic property or processes the polyolefin resin membranes with chemicals. However, because of simultaneously occuring chemical reactions, the molecular weight of polymer decreases and the structural integrity of the polyolefin membrane weakens. Further, due to the complexity of the processes involved, it is difficult to mass produce the polyolefin membranes having hydrophilic property.
Other methods for integrating hydrophilic property to the polyolefin membranes are further described by U.S. Pat. Nos. 4,346,142; 5,085,775; and 5,294,346. These methods use monomers of acrylic acid having hydrophilic property and polymers of polyethylene oxide by grafting them on to the surface of polymer membranes utilizing corona or plasma method. JP-A-8-31399 (unexamined published Japanese application) discloses a method of integrating both the hydrophilic and hydrophobic property to the polyolefin film surface by oxygen and carbon tetrafluoride gas utilizing plasma or sputter etching method. However, due to the plasma""s unique properties characterized by having a wide range of energy distribution and a high degree of environmental susceptibility, it is difficult to obtain an uniformed porosity. Further, obtaining a polyolefin membrane having excellent physical properties is made difficult by the degradation of its mechanical property due to the damage to the surface of the film caused by the reactions accompanying the method.
In view of the foregoing, it is an object of the present invention to provide a method for producing a microporous membrane having hydrophilic/hydrophobic properties and pores with uniform size and shape by irradiating energized ion particles to a polymer film under vacuum.
It is another object of the present invention to provide a method for producing a microporous membrane having high-density of pores.
It is yet another object of the present invention to provide a simple process method for producing a microporous membrane having hydrophilic property.
It is further object of the present invention to provide a method for producing a microporous membrane having hydrophilic property and excellent physical characteristics.
It is further object of the present invention to provide a microporous membrane prepared by the method.
According to the above methods of the present invention, a microporous membrane having excellent physical characteristics can also be obtained by irradiating ion particles of the microporous membrane produced from conventional methods.
The present invention will be described in detail below.
The present invention utilizes the principles for decreasing contact angle of hydrophilic solvents onto the surface of a polymer surface and increasing adhesion of the same by utilizing ion-beam irradiation.
Preparation of Precursor Film
A polymer film is obtained by using an extruder having a T-die or a tubular die, and is made from a polyolefin group consisting of polypropylene, high-density polyethylene, low-density polyethylene, and low-density linear polyethylene, because of cost and its low reactivity. Although an extrusion process can be carried out in a conventional extruding temperature, it is more preferable to carry out the process in temperature range of (polymer film melting point +10xc2x0 C.)xcx9c(polymer melting point +100xc2x0 C.). Extruding the polymer beyond this temperature range can lead to polymer degradation and consequently weaken its physical property.
The extruded polymer is drawn by using cast roll at 5xcx9c120 m/min in 10xcx9c150xc2x0 C. to obtain a precursor film, at draw down ratio of 10xcx9c400 and the freezing temperature of 10xcx9c120xc2x0 C.
Annealing
The precursor film is annealed at temperature range of (polymer film melting point xe2x88x9210xc2x0 C.)xcx9c(polymer melting point xe2x88x92100xc2x0 C.) for 10 sec. to 1 hour in order to obtain an elastic recovery over 40% at 25xc2x0 C. This annealing process increases both the elastic recovery and crystallinity of the precursor polymer film. Annealing at a temperature higher than this range may melt the polymer film, and annealing at a temperature lower than the range restricts the polymer movement and any significant increase in both the elastic recovery and crstallinity is very marginal.
Irradiation
The annealed precursor film is placed in a vacuum chamber under 10xe2x88x922xcx9c10xe2x88x928 torr, then both surfaces of the precursor film were irradiated with an ion-gun. The ion-gun was prepared by injecting a gas for generating energized ion particles to be used in irradiation by changing electrical current of ion-beam. Although, an irradiating distance from ion-gun to the surface of the precursor film of 5xcx9c100 cm is adequate, irradiating distance should be adjusted according to the vacuum pressure in the chamber. Such that, the irradiating distance should be 15xcx9c25 cm under a high vacuum of 10xe2x88x922xcx9c1031 3 torr, 25xcx9c55 cm under a high vacuum of 10xe2x88x923xcx9c10xe2x88x926, and 55xcx9c60 cm under a very high vacuum of 10xe2x88x926xcx9c10xe2x88x927. Any gas which has the capability of generating ion particles can be used by the ion-gun, however, electron, hydrogen, helium, oxygen, nitrogen, air, flourine, neon, argon, krypton, or N2O and their mixture compounds are also suitable for the purpose.
At this time, an energy level of the ion particles are set at 10xe2x88x922xcx9c10xe2x88x927 KeV and an irradiating amount is set at 102xcx9c1020 ions/cm2 by controlling a power supply device attached to the ion-gun. Irradiating ion particles with the above energy level and the amount, a microporous polymer film was obtained.
During or after irradiating with the ion-beam, a reactive gas in the amount of 0.5-20 ml/min can be applied to the polymer film for determining hydrophilic or hydrophobic property of the polymer film, according to a type of reactive gas applied. For providing a polymer film having hydrophilic property, it is preferable to use helium, hydrogen, oxygen, nitrogen, air, N2O, ammonia, carbon monoxide, carbon dioxide, or methane or their mixture compound; and to provide a polymer film having hydrophobic property, it is preferable to use flourine, carbon tetraflouride or their mixture compound. This process of determining hydrophilic or hydrophobic property of the polymer film can also be carried out after a final microporous membrane has been obtained.
Cold Stretching
The microporous polymer film obtained from the above irradiation process is subjected to a stretching process utilizing rolls or a biaxial stretcher by mono or biaxial stretching to increase the size of micro-pores formed in the polymer film. Here, the stretching is conducted at a temperature ranging from xe2x88x9220xc2x0 C. to (polymer melting point xe2x88x9240xc2x0 C.).
Hot Stretching
The microporous polymer film stretched from the cold stretching process was subjected to a further stretching process utilizing a roll or a biaxial stretcher by mono or biaxial stretching for obtaining micro-pores of desired size having mechanical property. Here, the stretching is conducted at a temperature of from (polymer melting point xe2x88x9240xc2x0 C.) to (polymer melting point xe2x88x925xc2x0 C.).
Heat Setting
The microporous film, hot stretched in a temperature below polymer melting point, having tension was then subjected to a heat setting under a tensioned state for maintaining the integrity of its stretched pores. Here, the heat setting is conducted at a temperature ranging from (polymer melting point xe2x88x9280xc2x0 C.) to (polymer melting point xe2x88x925xc2x0 C.).
A microporous membrane produced by the above methods of the present invention having a circular or an elliptical shape and a pore size of 0.005xcx9c10 xcexcm is suitable as a separator for lithium ion batteries. Additionally, a laminate membrane, produced by laminating a first microporous membrane produced by the methods of the present invention with a second microporous membrane produced by the conventional methods wherein the irradiating step has not been utilized, is also suitable as a separator for a lithium battery.
The above process of the present invention describes and provides a method for manufacturing a microporous membrane having excellent physical properties. The steps of the process can be deleted, changed or modified for providing a microporous membrane with a different or a desired property.
Herein below the preferred examples and comparative examples will be described in detail.